Angiogenesis and vasculogenesis are processes involved in the growth of blood vessels. Angiogenesis is the process by which new blood vessels are formed from extant capillaries, while vasculogenesis involves the growth of vessels deriving from endothelial progenitor cells. Angiogenesis and vasculogenesis, and the factors that regulate these processes, are important in embryonic development, inflammation, and wound healing, and also contribute to pathologic conditions such as tumor growth, diabetic retinopathy, rheumatoid arthritis, and chronic inflammatory diseases (see, e.g., U.S. Pat. No. 5,318,957; Yancopoulos et al. (1998) Cell 93:661–4; Folkman et al. (1996) Cell 87;1153–5; and Hanahan et al. (1996) Cell 86:353–64).
Both angiogenesis and vasculogenesis involve the proliferation of endothelial cells. Endothelial cells line the walls of blood vessels; capillaries are comprised almost entirely of endothelial cells. The angiogenic process involves not only increased endothelial cell proliferation, but also comprises a cascade of additional events, including protease secretion by endothelial cells, degradation of the basement membrane, migration through the surrounding matrix, proliferation, alignment, differentiation into tube-like structures, and synthesis of a new basement membrane. Vasculogenesis involves recruitment and differentiation of mesenchymal cells into angioblasts, which then differentiation into endothelial cells which then from de novo vessels (see, e.g., Folkman et al. (1996) Cell 87:1153–5).
Several angiogenic and/or vasculogenic agents with different properties and mechanisms of action are well known in the art. For example, acidic and basic fibroblast growth factor  (FGF), transforming growth factor alpha (TGF-α) and beta (TGF-β), tumor necrosis factor (TNF), platelet-derived growth factor (PDGF), vascular endothelial cell growth factor (VEGF), and angiogenin are potent and well-characterized angiogenesis-promoting agents. In addition, both nitric oxide and prostaglandin (a prostacyclin agonist) have been shown to be mediators of various angiogenic growth factors, such as VEGF and bFGF. However, the therapeutic applicability of some of these compounds, especially as systemic agents, is limited by their potent pleiotropic effects on various cell types.
Angiogenesis and vasculogenesis have been the focus of intense interest since these processes can be exploited to therapeutic advantage. Stimulation of angiogenesis and/or vasculogenesis can aid in the healing of wounds, the vascularizing of skin grafts, and the enhancement of collateral circulation where there has been vascular occlusion or stenosis (e.g., to develop a “biobypass” around an obstruction due to coronary, carotid, or peripheral arterial occlusion disease). There is an intense interest in factors that are well-tolerated by the subject, but that are of high potency in effecting stimulation of angiogenesis and/or vasculogenesis.